1. Field of the Invention
The invention relates to electrochemical devices, in particular of the type comprising at least one carrier substrate provided with a stack of functional layers comprising at least one electrically conducting layer and at least one electrochemically active layer. More particularly, the invention envisages electrically controllable systems having variable optical and/or energy properties, in glazing applications or mirror applications.
2. Discussion of the Background
The reason for this is the increasing demand for so-called “smart” glazing whose properties are capable of variation.
Thus, from the thermal standpoint, glazing whose transmission/absorption can be
Thus, from the thermal standpoint, glazing whose transmission/absorption can be varied within at least part of the solar spectrum allows the solar heat influx into rooms or passenger areas/compartments to be controlled when it is fitted as external glazing in buildings or as windows in means of transportation of the type comprising cars, trains, aircraft, etc., thus allowing avoidance of excessive heating of the latter in strong sunlight.
From the optical standpoint, the glazing allows the visibility to be controlled, and when it is installed as exterior glazing this makes it possible to prevent glare in strong sunlight. It may also have a particularly advantageous shutter effect, either used as exterior glazing or if used as interior glazing, for example for equipping internal partitions between rooms (offices in a building), or for isolating compartments in trains or aircraft.
There are many other applications: for example, glazing having variable-light transmission/reflection may be used to make rear-view mirrors, which can darken as required in order to prevent dazzling of the driver of the car. They may also be used for road sign panels, or for any display panel, for example so as to reveal the pattern or message only intermittently, in order to attract greater attention.
One particularly interesting application of systems having variable light absorption relates to display screens, in particular those with which televisions and computing hardware are equipped. The reason for this is that this type of glazing makes it possible to improve the contrast of the image, especially taking ambient brightness into account.
The interest which glazing of this type can arouse is the reason for the many systems which have already been studied.
Two systems are of more particular interest for the invention: viologenic systems and electrochromic systems.
Viologenic systems allow the transmission or absorption of glazing which incorporates them to be modified, essentially in the visible region. They generally comprise just one “active layer” based on polymer, on gel or on liquid comprising a so-called cathodic active material, such as viologenic molecules, together with a so-called anodic active material, such as dime thylferrocene or phenazines. Some examples of these are described in patents EP-0 612 826 and U.S. Pat. No. 5,239,406.
As is known, electrochromic systems include a layer of an electrochromic material capable of reversible and simultaneous insertion of ions and electrons and whose oxidation states corresponding to the inserted and ejected states have a distinct colour, one of the states having higher light transmission than the other, the insertion or ejection reaction being controlled by a suitable electrical supply. The electrochromic material, usually based on tungsten oxide, must therefore be placed in contact with a source of electrons, for example a transparent electrically conducting layer, and with a source of ions (cations or anions), for example an ionic conductive electrolyte.
It is moreover known that, in order to secure at least about a hundred switching operations, there must be associated with the layer of electrochromic material a counterelectrode which is itself capable of reversibly inserting cations, symmetrically with respect to the layer of electrochromic material so that, macroscopically, the electrolyte appears as a single ion medium.
The counterelectrode must consist of a layer which is either neutral in colour or at least transparent or with little colour when the electrochromic layer is in the coloured state. As tungsten oxide is a cathodic electrochromic material, that is to say a material whose coloured state corresponds to the most reduced state, the material used for the counterelectrode is generally an anodic electrochromic material based on nickel oxide or on 20 iridium oxide. It has also been proposed that the material used be optically neutral in the oxidation states concerned, for example cerium oxide or organic materials such as electronically conductive polymers (polyaniline, etc.) or Prussian blue.
A description of systems of this type is found, for example, in European Patents EP-0 338 876, EP-0 408 427, EP-0 575 207 and EP-0 628 849.
Currently, these systems may be arranged in two categories, according to the type of electrolyte which they use:                either the electrolyte is in the form of a polymer or of a gel, for example a polymer exhibiting protonic conduction, such as those described in European Patents EP-0 253 713 and EP-0 670 346, or a polymer exhibiting lithium-ion conduction, such as those described in Patents EP-0 382 623, EP-0.518+754 or EP-0 532 408,        or the electrolyte is an inorganic layer which is ionically conductive but electronically insulating; these systems are referred to as “all-solid” electro-chromic systems. For the description “all-solid” applied to an electrochromic system reference may be made to the European Patent Applications EP-0 867 752 and EP-0 831 360.        
There are other types of electrochromic systems. Mention may therefore be made of “all-polymer” electrochromic systems, where two electrically conducting layers are arranged on either side of a stack comprising a cathodic-colouring polymer, an electrically insulating but ionically conductive polymer (very particularly conductive for H+ or Li+) and finally an anodic-colouring polymer (such as polyaniline or polypyrrole).
Finally, there are also “active” systems in the sense of the invention which combine viologenic materials and electrochromic materials, for example having the sequence conducting electrode/inorganic layer or polymer having electrochromic properties/layers (liquid, gel, polymer) having viologenic properties/conducting electrode.
These systems comprising reversible insertion material(s) are of particular interest in the sense that they allow modification of absorption within a wavelength region which is wider than that for viologenic systems: they can absorb variably not only in the visible region but also, in particular, in the infrared region, and this can make them useful optically and/or thermally.
A point which is common to these different systems, described below by the term “active” systems is that their transmission/absorption state is con-trolled by applying a potential difference to their terminals, generally formed by two electrically conducting layers between which the electrochemically active layer(s) lie(s). When these systems are part of “active” glazing, the electrically conducting layers are preferably transparent (or at least one of them is transparent if the other is chosen to reflect in the visible region in a mirror application). The material needed when selecting the nature of these electrically conducting layers has therefore to be both sufficiently conductive and sufficiently transparent in the ranges of thickness usually encountered in the field of thin layers. The selection usually concentrates on a doped metal oxide material, such as fluorine-doped tin oxide (SnO2:F) or tin-doped indium oxide (ITO), which may be laid down on various substrates, either hot (in particular by pyrolysis on glass, as in the CVD method) or cold (vacuum techniques of cathodic sputtering type).
However, it has been found that, at thicknesses at which they are still transparent, layers based on this type of material are not fully satisfactory even though they do permit the functioning of active systems.
They are insufficiently conducting and increase the response time of the active systems on application to their terminals of an electrical supply appropriate to change their transmission/adsorption state (the state described below for greater simplicity by the term “coloration” state, even if the modification of properties also operates outside the visible region).
In addition to the fact that they reduce the switching speed of the systems (“switching” or “response time” being the period of time necessary for the entire active system to have changed its coloration state) the layers contribute to the creation of an edge phenomenon, i.e. non-uniformity in the change of state of the system within its surface, with a change in “coloration” in the sense of the invention which is almost immediate in the zones near to the current leads supplying the electrically conducting layers, arranged at the periphery of the systems, and which propagates progressively toward the centre of the surface of the active systems. Now, in certain applications, in particular glazing for buildings or automobiles, the final user generally desires the fastest possible response time and may, in addition, prefer a progressive, uniform change of coloration over the entire surface of the active glazing.
The object of the invention is therefore to improve the performance of the electrically conducting layers of the “active” systems defined below, and very particularly of “active” glazing comprising these latter, the improvement being aimed in particular at their electrical conductivity in association with their optical properties.